You seem to think that all that matters is the amount of light falling on a square micrometer of the sensor.

If you read the thread, you'll understand that my objective was to dispel the notion that f/1.8 on APS-C is in effect the same as f/2.8 on full frame. That notion is actually an unhelpful

Just the opposite. The end result is about the same, because it is mostly limited by purely optical considerations. No matter what technical solutions are employed between the subject and the final image on paper or on monitor.

and circuitous way of saying that APS-C sensors are, because of their typically smaller pixels, usually noisier than full-frame sensors - simple as that.

Nope. They aren't noisier "because of their typically smaller pixels". Given the same size of the lens' entrance pupil (i.e. the same amount of light captured by the lens from an area on the subject), the same angle of view and enough of pixel well depth, they all will provide about the same amount of noise on the final medium.

They are noisier because the entrance pupils of their smaller lenses are generally smaller than on the lenses that provide the same angle of view for larger sensor formats. There are physical limitations in making large entrance pupils for smaller focal lengths, as pupils bigger than f/0.5 are impossible even theoretically.

Nope, comparing shutter speeds of 70/2.8 to shutter speeds of 35/1.8 is not helping at all.

I didn't compare it specifically with 70mm, I compared it with a 24-70mm, because that was the comparison others made that started the debate.

While you should be comparing pictures/scenes shot with both and then presented on comparable mediums (screen or paper).

Otherwise the comparison is meaningless.

Which of those FAQs are you challenging?

Everything that relates to shutter speed.

Because it's not like people do (or at least should) pick their shutter speeds based on some fancy digits printed on their lenses. People should pick their shutter speeds based on the subject movements they need to freeze. If you cannot afford some part of the subject to move more than 5mm (in the subject plane) during the time of exposure, you will need to estimate the speed of this part (in mm/s) and divide it by 5 - then you will get (in 1/s) the lowest shutter speed you could afford to use in your shot, and then you will need to live with whatever amount of shot noise you are getting from the object get at this speed.

Now let's look how much noise we are actually getting. On our final image (printed or displayed on the monitor) we choose the smallest scale we need to resolve. It's historically - from the film era - called "circle of confusion" when reduced to length units in sensor plane; by at the moment we don't need it in length units, we need the solid angle that comprises this circle, the solid angle of the feature we need to resolve, the solid angle we need to get our photons from. And then the number of photons caught by this circle is the number of photons coming to our lens from that solid angle during the time of exposure - and then passing through the entrance pupil of our lens.

Then the absolute shot noise will be the square root of that number.And the relative shot noise will be (absolute shot noise)-1.

See? No fancy digits printed on the lens. No sensor size. Even no sensor's pixel size. Only the subject (under the lighting), the desired end result and the absolute effective lens opening (in square millimeters or similar area units). Once you get this fixed, you know what noise you will get for your shutter speed, and you know what shutter speed corresponds to your noise.

Of course, you may want to boost your sensor's ISO to reduce the in-camera noise (and the right setting of ISO usually makes in-camera noise irrelevant when you need higher shutter speeds). But that's what you can afford to control. What you cannot afford to control is the amount of photons coming from your subject.

Actually, the same considerations also hold true for background blur, DoF and - in some sense - bokeh. All what you would ideally see on the final picture was already determined when the light coming from the subject crossed the (virtual) entrance pupil (the rendering of diaphragm visible through the front lens), then anything lying after the entrance pupil only added its distortions to it.

When you want to apply the same to bokeh, you will need to take into consideration the non-uniformity (and dependence on the distance) of the entrance pupil's transparency for out-of-focus objects. It's uniform for lenses with well-corrected spherical aberrations, darker in center (and lighter at the blades) for nearer objects in under-corrected lenses and for farther objects in over-corrected lenses, and lighter in center (and darker at the blades) otherwise.

For example, an unobscured bright dot at infinity will be rendered on the final picture exactly like the entrance pupil moved (without change of its physical size) into the subject's plane of focus in the direction of that dot. It will be flatly lighted for lenses with well-corrected spherical aberrations, nicely round for lenses with under-corrected spherical aberrations and doughnut-shaped for lenses with over-corrected spherical aberrations. Other than that, nothing should happen behind the entrance pupil in a good lens (of course, there could be ghosts and stuff if the lens is not so good). So, if your shoot with EF 50/1.4 wide open, a bright dot on your (far enough) background will be rendered as a flat octagon of ~36 mm (in subject's plane of focus) diameter, no matter what subject you shoot, no matter what your magnification is (well, if it's true macro, there could be exceptions), no matter what sensor size your camera has.

You seem to think that all that matters is the amount of light falling on a square micrometer of the sensor.

If you read the thread, you'll understand that my objective was to dispel the notion that f/1.8 on APS-C is in effect the same as f/2.8 on full frame. That notion is actually an unhelpful

Just the opposite. The end result is about the same, because it is mostly limited by purely optical considerations. No matter what technical solutions are employed between the subject and the final image on paper or on monitor.

This doesn't advance the debate at hand.

Well, I could not just go and say that most guides that teach photography by numbers do it in the wrong way (not that their numbers are wrong, but they just focus your attention on secondary details, letting you fail to see what really happens with the picture). I needed some pretext for that.

Those guides pay too much attention to what happens in the camera, although it's what happens outside the camera is what forms your images. The camera just adds its own technical limitations. These technical limitations were big enough during the "35mm film rolls" era, big enough to limit your thinking about your subject and about your end result. Those limitations weren't as big for sheet film, those limitations aren't as big for current large photo sensors, you can focus now on what happens outside the camera, paying only secondary attention on what happens inside.

They aren't noisier "because of their typically smaller pixels". Given the same size of the lens' entrance pupil (i.e. the same amount of light captured by the lens from an area on the subject), the same angle of view and enough of pixel well depth, they all will provide about the same amount of noise on the final medium.

They are noisier because the entrance pupils of their smaller lenses are generally smaller than on the lenses that provide the same angle of view for larger sensor formats. There are physical limitations in making large entrance pupils for smaller focal lengths, as pupils bigger than f/0.5 are impossible even theoretically.

..In other words ISO 10,000 on crop is pretty much equal to ISO 25,600 on FF in terms of noise. So feel free to shoot smaller apertures on FF and use higher ISO's to get the same light gathering and noise....I see this Sigma lens as being a crop alternative to the 24-70 II on FF in just the same way as the 17-55 IS is a crop alternative to the 24-105L on F

I think everyone agrees that you would use the same ISO on both cameras.

I think everyone agrees that if you were physically cutting up film then crop f/1.8 would be the same as full frame f/1.8.

In the real world though, digital crop sensors are not simply full frame sensors with the edges chopped off. Manufacturers keep the same photo resolution -- ie shrink the photosites -- at the expense of high ISO noise. For an identical production process and same resolution ISO 3200 FF is not the same as ISO 3200 crop: for cameras made within a few years of each other, crop will always be noisier.

So in low light situations, how would you get the same quality images on crop as full frame? First you'd lower the ISO. But then it would be underexposed -- so you'd either lower shutter speed (something that every camera can do and has nothing to do with the lens) or ... open the aperture, just like rs said at the very start.

So while for a given area of a light sensetive material, ISO provides a set level of amplification, cameras do not have a consistent area that they absorb incoming light on

...

Simply put APS-C cameras have more dense sensors with more pixels, even with identical resolution. Because ISO is dependent on a given volume of light passing through a given area, if we increase the number of pixels in that area each pixel will have a stronger amplification level.

Let's say Canon releases the new 70D with a next-generation crop sensor... Now let's say this new sensor has noise performance as good as the 5D Mark II. (A stretch, but I think it's possible)

A crop camera with better ISO performance than the same manufacturer's previous full frame? Show where this has ever happened in history.

You're right, it's never happened in history. But maybe it will soon. (Yes, that's a big maybe!).

I'd guess that the noise performance of the new Rebel SL1 can't be too much worse than the original 5D's. I think the 7D Mark II has a good chance of having excellent noise performance. It will never beat the latest full frame bodies, but maybe it can give the previous generation of FF cameras a run for their money.

And what about using Sigma's f/1.8 zoom on a full frame camera? Sure, it's not designed for FF, but from the Korean site with the samples it appears to mount and work properly on a 5D Mark II (though with heavy vignetting at all FLs below 28mm). If you're willing to lose resolution by cropping out the vignetting, then you actually can achieve FF-level noise performance with this lens, and still shoot at an f/1.8 exposure. (AND get the full f/1.8 depth of field as well!)

I'm not trying to say that crop will be better than FF -- it never will be. I just want to clarify for the people who may not know or understand the facts: at the same ISO/ shutter speed, an f/1.8 crop exposure is always brighter than an f/2.8 FF exposure.

If crop performance can approach FF performance at only a fraction of the cost, then that's good news for photographers everywhere. That's why this new lens is a big deal and that's why I want to get one. (I don't even own any crop cameras right now!)

Let's say Canon releases the new 70D with a next-generation crop sensor... Now let's say this new sensor has noise performance as good as the 5D Mark II. (A stretch, but I think it's possible)

A crop camera with better ISO performance than the same manufacturer's previous full frame? Show where this has ever happened in history.

You're right, it's never happened in history. But maybe it will soon. (Yes, that's a big maybe!).

I'd guess that the noise performance of the new Rebel SL1 can't be too much worse than the original 5D's.

Based on the fact that the 5D was slightly ahead of the 7D (which if I'm not mistaken is the same sensor as SL1 but different firmware and/or QC?) I would think you're right -- dxo says the 5D is slightly ahead but I've tried shooting the same shot with both in low light and it's pretty hard to pick which is better (once you can actually get the 5D to focus!).

I'm not trying to say that crop will be better than FF -- it never will be. I just want to clarify for the people who may not know or understand the facts: at the same ISO/ shutter speed, an f/1.8 crop exposure is always brighter than an f/2.8 FF exposure.

In fairness, I've assumed that if you were wanting wide apertures it was usually for low light shooting -- if you're shooting low ISO and just want short depth of field then it really doesn't matter (both approach the human SNR perception limit)

I think everyone agrees that you would use the same ISO on both cameras.

I do not.

I'm quite happy to be shown wrong, but was extrapolating from what would happen with physical film, where physically cropping film does nothing to ISO (in which case ISO would indeed be the same on both full frame and crop).

I'm quite happy to be shown wrong, but was extrapolating from what would happen with physical film, where physically cropping film does nothing to ISO (in which case ISO would indeed be the same on both full frame and crop).

Cropping decreases the total light and increases the noise (because you enlarge).

The equivalent settings on FF requires the ISO to be multiplied by 2.56 (with the same QE anyway).

I'm quite happy to be shown wrong, but was extrapolating from what would happen with physical film, where physically cropping film does nothing to ISO (in which case ISO would indeed be the same on both full frame and crop).

Cropping decreases the total light and increases the noise (because you enlarge).

The equivalent settings on FF requires the ISO to be multiplied by 2.56 (with the same QE anyway).

I think we're talking about something different.

What I was talking about is a scenario i just tested:

Full frame camera (5d)- looking at a uniform white wall (possibly a cloudless sky is easier to make uniform?)- 50mm EF lens- f/4.0- 1/30sec- ISO for correct exposure is 1600

Same scene on an APSC camera (60d)- looking at a uniform white wall- 50mm EF lens [yes, it's cropped and not the same image -- that's why I specified a uniform wall]- f/4.0- 1/30sec- ISO for correct exposure is still 1600. Histograms are virtually identical centered slightly above 50%. ISO 4000 is very much overexposed.

In this situation, a crop camera is the same as physically cropping film and the ISO is unchanged.

The fact that the total light reaching the crop sensor is less doesn't change the camera's ISO setting, the fact that less total light reaches the crop sensor changes the amplifiers to meet the ISO sensitivity definition.

Edit: changed the scenario settings to be real settings rather than hypothetical.

More amplification in the crop means more noise; when you say "equivalent settings" I assume you actually mean "equivalent settings to reach the same SNR"?